Elaboration of forging conditions on the basis of the precipitation analysis of MX-type phases in microalloyed steels

Abstract

The paper presents analysis of the precipitation of MX-type (M—microalloying element, X—interstitial element) phases in austenite of constructional steels microalloyed with Nb, Ti, V and B, assigned for production of forged elements with the use of thermo-mechanical treatment. The calculations were conducted basing on the dependence of solubility product of MX-type phases in austenite as a function of temperature. The analysis was done on the basis of a simplified thermodynamic model for equilibrium conditions, assuming that individual MX-type phases are soluble in austenite. The solubility of TiN, TiC and NbC as a function of temperature as well as the simplified temperature sequence of precipitation of these phases have been determined. The effect of austenitizing temperature in a range from 900 to 1200°C on grain size of prior austenite was investigated to verify the analysis. A grain growth corresponds well with a course of a precipitation process, though the limitation of the current model arising from a lack of consideration of complex carbonitrides and the kinetics of a precipitation process should be taken into account. The studies provide the basis for proper design of manufacturing process of thermo-mechanical treatment for high strength forged elements of microalloyed steels.